I don't have a problem with this at all. I can understand how they'd be upset that someone takes their software, makes a custom map that is enjoyable, and then someone makes a straight copy of it after a few years and makes a fat wad of cash to boot. Games aren't the only place where things like this happen, a lot of jobs stipulate that anything you create while employed there is the companies property.
i like valve's way more they are encouraging people to make maps and actually paying them for rights of map if they are very good